The present invention relates to a method for treating polyetherimide surfaces to improve their adhesion characteristics to metal, such as copper, after the metal has been electrolessly deposited onto such molded or extruded polyetherimide surfaces.
Prior to the present invention, as shown by co-pending application Ser. No. 103,618, improved adhesion was achieved on a polyetherimide surface by treating the polyetherimide surface with an adhesion promoter such as thiourea. However, it has been found that thiourea can discolor copper inserts in a molded polyetherimide circuit board resulting in poor bonding between the copper insert and the electrolessly plated copper. In co-pending application Ser. No. 207,462, improved adhesion of the polyetherimide surface is achieved by initially modifying the surface of the polyetherimide with a mild etchant, such as sulfuric acid, contacting the modified surface with a basic solution followed by contacting the treated surface with a cationic surfactant which effects the removal of a residual film formed on the surface after the sulfuric acid step. Removal of the residual film or white residue is essential to optimize the adhesion of any subsequently applied electrolessly deposited metal onto the plastic substrate. Although effective results have been achieved by the employment of a cationic surfactant, it has been found that the concentration of the white residue resulting from sulfuric acid immersion followed by a water rinse, can actually increase in the solubilizing bath, since the white residue is merely solubilized not chemically altered. As a result, treatment of the polyetherimide surface with the aqueous cationic surfactant can eventually result in reduced adhesion of the electrolessly deposited metal on the polyetherimide surface.